Thermoplastic vinylidene fluoride (VDF) polymers are fluoropolymers well known for their outstanding properties, including barrier properties (e.g. against oxygen and/or water), chemical resistance, mechanical properties, weatherability; it is generally understood that these mechanical properties are correlated with the crystalline phase of said thermoplastic VDF polymers, materialized in terms of heat of fusion and melting temperature of said polymer.
Also, high melting point VDF polymers are generally preferred when high temperature rating is required, e.g. for applications in extreme temperature conditions.
It is also widely recognized that certain uses of VDF polymers, in particular of VDF polymer films, require the material to be transparent to visible light and to be translucent, so as no deviation of incident light occurs.
Nevertheless, transparency and translucency are properties which can be achieved through behaviour of the amorphous phase of said VDF polymers.
General approaches for obtaining transparent and translucent articles are based on VDF copolymers, e.g. by incorporation of fluoromonomers like hexafluoropropylene (HFP) or chlorotrifluoroethylene (CTFE). These modifying comonomers are well-known for their ability of preventing crystallization of otherwise regular sequences of VDF recurring units in the polymer chain, by introducing ‘defects’, so that copolymer containing the same have their crystallinity substantially reduced with respect to VDF homopolymers.
While thus amorphous character by means of these approaches is enhanced, with corresponding improvement of transparency and translucency, otherwise barrier properties, chemical resistance, mechanical properties, weatherability and thermal rating are consequently negatively affected.
Now, several fields of use exist wherein any of above mentioned properties combined with outstanding transparency and look-through capabilities are required. As non limitative examples, mention can be made of protective films for photovoltaic modules, transportation, industrial and food packaging, pharmaceuticals storage and packaging and the like.
There is thus still a current shortfall in the art for thermoplastic VDF polymer compositions offering an improved compromise between barrier properties, chemical resistance, mechanical properties, weatherability and thermal rating, from one side, and transparency and translucency, on the other side.
On the other side, compositions wherein thermoplastic VDF polymers have been combined variously with certain ethylene-chlorotrifluoroethylene (ECTFE) polymers have been described in the art.
Thus, U.S. Pat. No. 5,908,704 (NORTON PERFORMANCE PLASTICS CO) Jan. 6, 1999 is directed to THV-based compositions for protective glazing applications; the addition of ECTFE in THV in amounts of 1 to 30% is taught to yield semi-opaque films having a haze value of 4-25%. Specific working embodiments are provided, pertaining to THV-ECTFE compounds comprising from 10 to 90% ECTFE and possessing haze values (under the form of films) of 10 to 50%, thus corresponding to non translucent items. ECTFE resin used is HALAR® 300 ECTFE resin, which is known to possess a melting point of 240° C.
Similarly, WO 01/38076 (LITHIUM POWER TECHNOLOGIES INC) May 31, 2001 is directed, inter alia, to certain VDF polymer-based compositions incorporating therein another fluoropolymer; among a long list, mention is also made of ECTFE polymers. No specific example of a blend of PVDF and ECTFE is provided. Among embodiments provided therein, an approach is directed to the improvement of high temperature features of PVDF by incorporation of said second fluoropolymer: to this aim, the second fluoropolymer is selected among those which are temperature resistant up to 250° C., while PVDF is taught as being merely temperature stable up to 175° C.
WO 2009/026284 (3M INNOVATIVE PROPERTIES) Feb. 26, 2009 discloses solvent-coating compositions comprising a first fluoropolymer soluble in a solvent and a second fluoropolymer insoluble in said solvent. Among a long list of ‘soluble’ fluoropolymers, mention is made of VDF polymers; among a long list of ‘insoluble’ fluoropolymers, ECTFE is mentioned but no specific example is directed to this combination. Matching of refractive index among the two fluoropolymers is taught as guidance for obtaining transparent films.
None of the prior art discloses nor fairly suggest a thermoplastic VDF polymer composition possessing an improved compromise between transparency/translucency properties and barrier properties, chemical resistance, mechanical properties, weatherability and thermal rating.